ETL 1110-2-540
30 Sep 96
(3) Even where considerable data are available for initial
However, if the hydrologic forecasts were very sensitive to
calibration, the first forecast results must be used with care.
even slight changes in the meteorological forecasts, decisions
Each rain gage network has its own unique characteristics as
can be made but decision-makers must be ready to alter
to how well its measurements represent areal rainfall. These
strategies quickly should developing conditions warrant.
characteristics become imbedded implicitly in hydrologic
model parameter estimation during the calibration process. If
the operational network is different from the network used in
selected plan cost is offset by reasonably attainable benefits.
calibration, forecast errors may result since the operational
Each emergency response action taken as the result of
network will have its own set of characteristics and abilities to
advanced planning and increased warning time has direct
represent areal rainfall. Therefore, frequent recalibration is
consequences in terms of derived benefits. The anticipated
often required as the database for the operational network
benefits from a flood warning - preparedness program can be
grows.
categorized and associated with the contributing actions as
noted in Table 4-6.
(4) Because of these problems, the hydrologic fore-
casting component can be the least reliable portion of the
Table 4-6
flood warning system in its early stages.
This is
Example Benefit Categories
unfortunate. Once the hydrologic forecast model is installed,
Category
Contributing Action
it generally becomes the focus of attention. If it does not
Reduced threat to life
Barricades, evacuations, rescues, public
work well, skeptics will say that the entire flood warning
awareness
system does not work. If this happens early on before users
Reduced property loss
Removal or elevation of residential and
have a chance to understand the system and gain confidence
commercial structure contents and
in it, the system may never have a chance to succeed. System
vehicles
credibility is critical, since many life and death decisions may
Reduced social disruption
Traffic management, emergency ser-
be made based on it.
vices, public awareness
(5) To avoid this problem, a staged implementation of
Reduced health hazards
Evacuations, public information, emer-
gency services
the flood warning system might be more appropriate. The
basic data acquisition system including measurement, trans-
Reduced disruption of
Utility shutoffs, emergency services,
mission, and data handling at the base station is the portion of
public services
supplies, inspection supplies, inspection,
public information
the system subject to the least amount of uncertainty. It is the
easiest to understand and gain confidence in. A minimum
Reduction in inundation
Flood fighting, temporary flood damage
number of rain- and stream gages may be installed initially
reduction measures, technical assistance
and additional gages added later as gaps in the data become
apparent. By implementing this portion first and allowing
Plan enhancements should be analyzed and evaluated based
users to gain confidence, the foundation is being laid for the
on general improvements over the existing condition, the cost
future addition of gages and hydrologic modeling, if
of each component, and the potential contributions to
appropriate. The database gets a chance to develop so that
reducing flood damages and preventing the loss of life. A
calibration can take place using the same network that will be
strict economic analysis including explicitly quantified flood
used operationally. Should the models produce mediocre
damage reduction benefits is not required. Reasonable
results at first, the underlying confidence remains and users
estimates of flood damage reduction benefits based on specific
will realize that it is only the hydrologic modeling portion that
actions that can be accomplished in the time afforded and
needs improvement.
within community resources are required. The warning time
made available from enhanced flood warning establishes the
(6) Once the hydrologic forecast model is properly
actions that achieve beneficial results. There are reasonable
calibrated and becomes integrated into the forecast system, it
limits to what can be achieved in a given amount of time.
can become a vital decision-making tool. Forecasts can be
The "day curve," shown in Figure 4-4, can be used as a
made using measured rainfall and the results can be evalu-
check of the reasonableness of the stated flood loss reduction
ated. More importantly, alternative scenarios of future rainfall
benefits. This is accomplished by making a comparison
patterns can be tested. These results can be evaluated and
between the estimated benefits and the maximum indicated by
used to test the sensitivity of forecast results to meteorological
the curve. It should be noted that the curve shown in the
forecasts. For example, if widely varying meteorologic
figure is a generic curve. If possible, a site specific curve
inputs cause little or no difference in hydrologic model results,
should be developed to more accurately reflect study area
decisions could be made with high confidence levels.
conditions.
4-17
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